Platelets fulfill essential functions in hemostasis, inflammation, angiogenesis, and other processes. Many of these functions require regulated secretion from three types of storage compartments - granules, dense granules, and lysosomes. Many hemorrhagic and thrombotic disorders are caused by dysregulation of granule formation and/or content release. In particular, bleeding diathesis in Hermansky-Pudlak syndrome (HPS) results from an absence of detectable platelet dense granules and consequent defects in activation-dependent release of calcium and ADP, which normally promote thrombus formation. Despite the importance of platelet granules, the mechanisms underlying their formation and membrane dynamics within megakaryocytes (MKs) and/or their derived platelets are largely uncharacterized, and the pathways regulated by the genes that are defective in the nine characterized HPS variants are not known. In other cell types, HPS-associated genes regulate vesicular transport processes required to deliver resident integral membrane proteins from early endosomes to lysosome-related organelles - cell type-specific intracellular storage compartments like and dense granules. We and our collaborators have recently identified two integral membrane proteins - SLC35D3 and VMAT2 - as candidate dense granule-specific cargoes that can serve as markers to study dense granule biogenesis. Surprisingly, these two cargoes localize to early endosomes in MKs from both wild-type and HPS model mice. Nevertheless, SLC35D3 is destabilized in platelets from the same HPS models. Based on these and other data, we hypothesize that dense granule integral membrane proteins are delivered from early endosomes to immature dense granules at a late stage of differentiation of platelets from MKs. We further hypothesize that dense granule membranes are not static but are constantly remodeled in active platelets, perhaps providing an avenue for delivery of pharmacological agents to platelet dense granules. Finally, preliminary data suggest that activation-induced content release is impaired not only from dense granules but also from granules and lysosomes in HPS model platelets, both in vivo and ex vivo. Since content release requires fusion of the granule membrane with the plasma membrane, we hypothesize that this secretory defect in HPS models reflects impaired cargo delivery of the fusion machinery to granules and lysosomes. We will test these hypotheses in the following Specific Aims: 1. To assess whether HPS-associated proteins regulate the dynamic localization of candidate dense granule membrane proteins in platelets. 2. To assess whether HPS impacts maturation of dense granules by fusion of distinct precursor organelles. 3. To test whether and how HPS subtypes impact -granule and lysosome secretion from platelets.